18 Methods in Plant Histology 



osmic acid, thus securing the advantages of a very rapid killing 

 without the blackening which results from a prolonged treatment 

 with a solution containing osmic acid. 



Probably no process in microtechnic is in more urgent need of 

 improvement than this first step of killing and fixing. Nearly all 

 of our formulae are merely empirical, for very few botanists are 

 expert chemists, and those who have the requisite knowledge of 

 chemistry are interested in physiological problems rather than in 

 microtechnic. The principal ingredients of the usual killing and 

 fixing agents are: alcohol, chloroform, chromic acid, acetic acid, 

 osmic acid, formic acid, picric acid, sulphuric acid, platinum chloride, 

 iridium chloride, corrosive sublimate, and formalin. We shall con- 

 sider first : 



THE ALCOHOLS 



a) Ninety-five Per Cent Alcohol.- -This is in quite general use 

 for material which is needed only for rough work. It is extremely 

 convenient, since it kills, fixes, and preserves at the same time and 

 needs no changing or washing. It really has nothing to recommend 

 it for fine work. It causes protoplasm to shrink, but cell walls 

 usually retain their position, so that 95 per cent alcohol will do for 

 freehand sections of wood and many herbaceous stems; but even 

 freehand sections of tender stems, like geraniums and begonias, will 

 look better if better reagents are employed. Alcohols weaker than 

 95 per cent are not to be recommended as fixing agents, although 70 

 per cent alcohol, or even 50 per cent, will preserve material for habit 

 work. The time required for fixing in 95 per cent alcohol is about 

 the same as for absolute alcohol. The subsequent treatment is the 

 same, except that material to be imbedded in paraffin or celloidin 

 must be dehydrated in absolute alcohol. Material preserved in 

 weaker alcohols and intended only for habit study may be kept in the 

 the reagent until needed for use. Unless some glycerin be added, 

 material left in 95 per cent alcohol becomes very brittle. Stems, 

 roots, and similar objects may be kept indefinitely in a mixture of 

 equal parts of 95 per cent alcohol and glycerin. 



Methyl alcohol, or wood alcohol as it is commonly called, serves 

 equally well. 



